A borehole is conventionally drilled during the recovery of hydrocarbons from a well, the borehole typically being lined with a casing. Casings are installed to prevent the formation around the borehole from collapsing. In addition, casings prevent unwanted fluids from the surrounding formation from flowing into the borehole, and similarly, prevents fluids from within the borehole escaping into the surrounding formation.
Boreholes are conventionally drilled and cased in a cascaded manner; that is, casing of the borehole begins at the top of the well with a relatively large outer diameter casing. Subsequent casing of a smaller diameter is passed through the inner diameter of the casing above, and thus the outer diameter of the subsequent casing is limited by the inner diameter of the preceding casing. Thus, the casings are cascaded with the diameters of the casing lengths reducing as the depth of the well increases. This gradual reduction in diameter results in a relatively small inside diameter casing near the bottom of the well that could limit the amount of hydrocarbons that can be recovered. In addition, the relatively large diameter borehole at the top of the well involves increased costs due to the large drill bits required, heavy, equipment for handling the larger casing, and increased volumes of drill fluid that are required.
Each casing is typically cemented into place by filling cement into an annulus created between the casing and the surrounding formation. A thin slurry cement is pumped down into the casing followed by a rubber plug on top of the cement. Thereafter, drilling fluid is pumped down the casing above the cement that is pushed out of the bottom of the casing and into the annulus. Pumping of drilling fluid is stopped when the plug reaches the bottom of the casing and the wellbore must be left, typically for several hours, whilst the cement dries. This operation requires an increase in rig time due to the cement pumping and hardening process, that can substantially increase production costs.
It is known to use a pliable casing that can be radially expanded so that an outer surface of the casing contacts the formation around the borehole. The pliable casing undergoes plastic deformation when expanded, typically by passing an expander device, such as a ceramic or steel cone or the like, through the casing. The expander device is propelled along the casing in a similar manner to a pipeline pig and may be pushed (using fluid pressure for example) or pulled (using drill pipe, rods, coiled tubing, a wireline or the like).
Lengths of expandable casing are coupled together (typically by threaded couplings) to produce a casing string. The casing string is inserted into the borehole in an unexpanded state and is subsequently expanded using the expander device, typically using a substantial force to facilitate the expansion process. However, the unexpanded casing string requires to be anchored either at or near an upper end or a lower end thereof during the expansion process to prevent undue movement. This is because when the casing string is in an unexpanded state, an outer surface of the casing string does not contact the surrounding borehole formation or an inner face of a pre-installed casing or liner (until at least a portion of the casing has been radially expanded), and thus there is no inherent initial anchoring point.
Slips are conventionally used to temporarily anchor the unexpanded casing to the borehole during the expansion process. Slips are generally wedge-shaped, steel, hinged portion that provide a temporary anchor when used. Slips are actuated whereby the wedge-shaped portions engage with the surrounding borehole formation or a casing or liner.
However, the mechanical configuration of slips often causes damage to the casing or liner. In some cases, the damage causes the slip to fail due to a loss of mechanical grip. Slip-type devices in open-hole engaging formation are often prone to slippage also.